Add a button to create the missing val in 404 errors

[pomdtr]

Example usecase:

I'm working on https://www.val.town/v/pomdtr/val_town_by_example_parse, and I want to write a test for it.

I would like to just change the url to https://www.val.town/v/pomdtr/val_town_by_example_parse_test, and then click on a button to create the val.

[stevekrouse]

This is a great idea! Thanks!